docs: def init_app(self, app):
          """
          This callback can be used to initialize an application for the
          use with this prometheus reporter setup.
  
          This is usually used with a flask "app factory" configuration. Please
          see: http://flask.pocoo.org/docs/1.0/patterns/appfactories/
  
          Note, that you need to use `PrometheusMetrics(app=None, ...)`
          for this mode, otherwise it is called automatically.
  
          :param app: the Flask application
          """
  
          if self.path:
              self.register_endpoint(self.path, app)
  
          if self._export_defaults:
              self.export_defaults(
                  self.buckets, self.group_by,
                  self._defaults_prefix, app
              )